The agouti gene is very important in the determination of
horse color because it regulates the distribution of the black pigment. The
effects of the Agouti (A) gene are therefore only visually obvious in the
presence of the Black (E) allele. The dominant Agouti allele -- A --
restricts the distribution of black pigment in hair to a points pattern (so
any black that shows up on the horse will be found mostly on the legs, mane,
tail and ear rims). The Agouti color distribution is found on horses that
are bay or a dilution of bay (buckskin, perlino, amber champagne, zebra dun,
The recessive allele -- a -- causes the distribution of black pigment over
the entire horse and, when homozygous in the presence of the E, produces a
uniformly black horse.
Regardless of what color it is bred to, any horse who is homozygous for
the Agouti gene i.e., has a double Agouti AA allele -- will NEVER
produce a solid black or smoky black foal. In fact, a horse who is
homozygous for Agouti will always produce foals, either with the black
restricted to the points (bay, buckskin, perlino or amber champagne), or with
no black at all (i.e., a chestnut, palomino or cremello foal is possible if
both parents carry an "e" gene) who also carries an Agouti gene. A
horse who is homozygous for the Agouti gene will ALWAYS pass one Agouti gene
to every single foal that the horse produces.
A Perlino with a double Agouti will never sire a Smoky Black or Smoky Cream
foal because those colors cannot have an Agouti gene.